This happened to me after upgrading the kernel from 6.1 to 6.6. After the upgrade, I rebooted the machine without running grub-install first, which rendered the machine unbootable.

The fix is to reinstall GRUB with the help of the Gentoo Live USB:

# mount /dev/nvme0n1p3 /mnt/gentoo
# cd /mnt/gentoo
# mount -t proc /proc /mnt/gentoo/proc
# mount --rbind /sys /mnt/gentoo/sys
# mount --rbind /dev /mnt/gentoo/dev
# mount --bind /run /mnt/gentoo/run
# chroot /mnt/gentoo /bin/bash
# . /etc/profile
# mount /dev/nvme0n1p1 /boot
# grub-install --efi-directory=/boot
# grub-mkconfig  -o /boot/grub/grub.cfg
# exit

Steps taken from Gentoo Forums, I only changed the disk to /dev/nvme0n1, as my H3 uses a NVMe SSD to boot from:
https://forums.gentoo.org/viewtopic-p-8816669.html#8816669